Is America's success going to our heads?

The size of Americans' skulls has been growing since the 1880s, researchers… (AFP / Getty Images )

America's rousing success over the past two centuries may be going to our heads, Tennessee researchers say. A study of skull sizes with specimens ranging from the mid-1800s to the mid-1980s indicates that the average head size of Americans has increased during the period. The researchers are unable, however, to pinpoint a precise reason for the change, and are unsure whether it is due to evolution or lifestyle changes. But they noted that average heights are also increasing, and that has frequently been attributed to better nutrition and improved medical care.

Anthropologist Lee Jantz of the Forensic Anthropology Center at the University of Tennessee and his colleagues studied 1,500 skulls of Caucasian Americans. Jantz reported at a meeting in Portland, Ore., of the American Assn. of Physical Anthropologists that the skulls have become larger, taller and narrower as seen from the front, and faces have become significantly narrower and higher.

The team found that in men the average height of the skull from base to top has increased by 8 millimeters (0.3 inches) during the period, while the capacity has grown by by 200 cubic centimeters, about the size of a tennis ball. In women, the corresponding increases are 7 mm and 180 cc. That is a 6.8% increase in skull height, compared with a 5.6% increase in body height during the period and a 2% increase in femur length. Moreover, the researchers said, skull height has continued to increase, while body height has plateaued.

The team also observed that people are maturing earlier, as indicated by the earlier closing of a bone structure in the skull called the spheno-occipital synchondrosis. In the past, this opening -- which allows the skull to expand to accommodate the growing brain -- was thought to close around age 20. Today, the team found, the bone is fusing at age 14 in girls and 16 in boys.

"The varieties of changes that have swept American life make determining an exact cause an endlessly complicated proposition," Jantz said. "It likely results from modified growth patterns because of better nutrition, lower infant and maternal mortality, less physical work, and a breakdown of former ethnic barriers to marriage. Which of these is paramount we do not know."